Method of pulverized coal firing a steam generator and controlling steam temperature



Dec. 5, 1967 w. L. LIVINGSTON 3,356,075

METHOD OF PULVERIZE'D COAL FIRING A STEAM GENERATOR AND CONTROLLING STEAM TEMPERATURE Filed Oct. 12, 1965 2 Sheets-Sheet 1 WILLIAM L. LIVINGSTON INVENTOR.

BY A v ATTORNEY Dec. 5, 1967 w. L. LIVINGSTON 3,356,075

METHOD OF PULVERIZED COAL FIRING A STEAM GENERATOR AND CONTROLLING STEAM TEMPERATURE Filed Oct. 12, 1965 2 Sheets-Sheet 2 STOICH/OMETR/C VALUE INTROPUCEP AT FIRST ZONE 0F AVERA 6E ABSO/ZPf/ON FURNACE HEIGHT W/LL IAM L. u v/MssToA/ F76 2 INVENTOR.

United States Patent 3,356,075 METHOD OF PULVERIZED COAL FIRING A STEAM GENERATOR AND CONTROLLING STEAM TEMPERATURE William L. Livingston, Huntsville, Minm, assignor to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed Oct. 12, 1965, Ser. N 495,286 8 Claims. (Cl. 122479) ABSTRACT OF THE DISCLOSURE An improved process for the operation of a steam generator wherein pulverized coal is introduced into a lower zone in the furnace at a fuel-rich mixture between 40 and 80 percent of theoretical. Above and spaced from this zone the remaining air for complete combustion is supplied. The combustion gases pass over superheating surface and a control of the superheat temperature is provided by adjustably proportioning the air introduction between the lower and upper furnace locations.

This invention relates generally to the art of steam generation and has particular relation to an improved method of firing a steam generator by means of pulverized coal and of controlling the temperature of the steam with varying load.

In accordance with the improved process of the invention there is provided a steam generator which is of high capacity and which has a relatively large elongated furnace. The furnace is fired with pulverized coal and rather than burning the pulverized coal in a particular zone or region, there is provided What may be termed twostage firing with all of the pulverized coal being introduced into the first stage While the air is divided between the first and second stage. To explain the invention reference will be had to a typical steam generator as employed in the utility field wherein there is utilized an elongated upright furnace. Firing mechanism is associated with the lower region of the furnace and in this region all of the coal, but only a portion of the air, is introduced. The pulverized coal is introduced in typical manner such that suspension firing is provided. The air introduced into this lower region is regulated such that it varies within the limits of above 40% and below 80% of the stoichiometric air requirement for the fuel. It has been found that with this very rich fuel supply stable firing is obtained with temperatures in the furnace above 1800 F., actually more stable than air-rich. It is further found that large quantities of CO gas are produced in this first zone with these temperatures and with this fuelrich firing. The gases thus produced in the lower region are passed up through the furnace and at a region spaced well above what may be termed the first zone of firing, there is established a second zone at which only air is introduced into the furnace to react with the flammable gases produced in the first zone. Only sufiicient air is introduced to provide complete combustion, and it is found with this arrangement the total air requirement is relatively small as, for instance, only 3% excess air being required to provide complete combustion of the combustible constituents. It is further found with this arrangement that a relatively uniform temperature as compared with the ordinary firing methods for pulverized coal is obtained within the furnace and the slagging conditions are very much improved with there being considerably less slag developed and in some instances no slag at all where slag was previously formed. This, in turn, reduces the corrosion that takes place with regard to the heat exchange surface in the furnace. The more 3,356,675 Patented Dec. 5, 1967 uniform temperature in the furnace lowers the maximum heat transfer rate which, in turn, enables the use of tubes of less wall thickness along the furnace walls.

With this firing process an appreciable amount of con trol of reheat or superheat temperature may be provided in an arrangement wherein the combustion gases that are generated in the furnace traverse the superheating and/or reheating surface. This is brought about by regulating the proportioning of air introduced between the first and the second zones. This may be achieved within the limits of the air requirements of the first zone, i.e., the above 40 and below stoichiometric amount. The less air introduced into the first zone requires that more air be introduced in the second zone with the ultimate result being that the temperature of he gases leaving the furnace increases which will, in turn, increase the temperature of the steam which is heated by these gases.

It is accordingly an object of this invention to provide an improved process for operation of a steam generator.

Another object of this invention is to provide an improved method of firing a steam generator with pulverized coal.

Still another object of this invention is to provide an improved system in a steam generator fired with pulverized coal wherein regulation of steam temperature may be achieved.

Other and further objects of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization in such a manner as to attain the results desired, as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawings wherein:

FIGURE 1 is a vertical sectional view, schematic in nature, depicting a high capacity steam generator equipped to utilize the improved system of this invention;

FIGURE 2 is a graphic representation showing the more uniform temperature conditions that are achieved within the furnace with the system of the invention; and

FIGURE 3 is a graphic representation depicting the control that can be achieved with relation to the temperature of the gases egressing from the furnace.

Referring now to the drawings, wherein like reference characters are used throughout to designate like elements, the illustrative and preferred embodiment of the invention depicted therein includes a high capacity steam generator that has a furnace 10 the combustion gas outlet 12 of which is located at the upper end. There extends from this outlet a horizontal gas pass 14 which, in turn, connects with the vertical gas pass 16. This latter gas pass is connected at its lower end with the regenerative air heater 1% and there extends from this air heater the duct 24) which conveys the combustion gases to a stack or other suitable location. The steam generator is provided with various heat exchange surface in the form of tubes through which the working medium of the generator is conveyed with there being produced superheated steam that is conveyed to a high pressure turbine 22. This heat exchange surface may include the economizer 24 which is supplied with the working medium via the feed pump 26. The fluid is conveyed from this economizer through the various tubes which include tubular members 28 which line the four walls of the furnace with the furnace being generally rectangular in transverse configuration. The steam generator niay be of the subcritical or supercritical type and as disclosed the steam is eventually received in the supply header 3t in a predetermined superheated condition and is conveyed therefrom to the turbine 22.

In the illustrative embodiment there is provided in the gas pass 14 a reheater 32 and the exhaust from high pressure turbine 22 is received within this reheater and is heated to a desired value after which it is conveyed to low pressure turbine 36. The exhaust from this low pressure turbine is conveyed to a condenser and to the various feedwater heaters as is conventional in a steam power plant system with this equipment not being shown.

The furnace is fired by pulverized coal and for this purpose there is provided at the lower region of the furnace pulverized coal burners 38. These burners receive pulverized coal through the transport pipes 40 from a suitable grinding mill or the like with the coal being transported in an air stream through these pipes to the burners 38. Heated air is supplied adjacent the burners 38 by means of the fan 42 which has its discharge connected with duct 44 within which there is interposed the air heater 18. This duct is connected with the windbox 46 at the lower region of the furnace and there is provided at the inlet of this windbox the control damper 48. The fuel and air are introduced into the lower region of the furnace via the burners 38 and windbox 46 so that they react in the zone identified as 48 so as to produce heat with the proportioning of the fuel and the air being such that an extremely fuel-rich (relative to stoichiometric) mixture is provided in this zone 48 with the air being maintained below 80 and above 40% of the stoichiometric amount required for the fuel. The relationship of the heat adsorption in this zone together with the fuel and air introduction therein is maintained so that the temperature in this zone 48 is 1800" F. or above. The effluent from the zone 48 is conveyed up through the furnace through the zone 50 and in this zone air is introduced into the furnace from the windbox 52 which is also connected with the duct 48 and with there being a control damper 54 in this interconnection. Only air is introduced into the furnace at this zone 50 and only sufficient air is introduced so as to provide for complete combustion of the constituents in the efiiuent passing up through the furnace from the zone 48. The combustion gases continue to pass up through the furnace and egress through the furnace outlet 12 and traverse the heat exchange surface in the horizontal gas pass 14, i.e., the reheater 32 and the heat exchange surface in the vertical gas pass 16 eventually passing through the air heater 18 and then to the stack.

With this very fuel-rich firing it is found that a very stable flame is obtained in the furnace considerably more stable than in the normal pulverized firing coal system wherein all of the air requirement is introduced with the fuel at the particular zone of combustion. It is further found that a considerably more uniform temperature is obtained in the furnace, and the slaggin-g problems heretofore experienced are considerably diminished if not completely eliminated. With the diminution of these slagging problems, there is a decrease in the corrosion and metal wastage obtained in the furnace which is associated with slagging conditions. Because of the considerably less slag that is formed, the amount of heating surface required is substantially less than with the conventional firing method.

Heretofore the firing of pulverized coal in the rich mixture utilized in zone 48 with the present invention was steadfastly avoided and regarded as a dangerous and very undesirable condition. I have shown, on the contrary, that this firing can be utilized in a process with advantageous results and wherein complete combustion is obtained with less excess air than ordinarily is required and wherein improved flame stability is obtained over that with conventional pulverized coal firing and wherein heat distribution and slag conditions within the furnace are substantially improved.

FIGURE 2 ilustrates this more uniform temperature distribution wherein the abscissa represents furnace height and the ordinate represents percentage of average absorption. With the conventional firing the percentage of absorption was predominant at the firing zone where the highest temperature was obtained while with the improved process of this invention the percentage of absorption is considerably more uniform as a result of the uniform temperature conditions in the furnace.

It is theorized that these improved conditions are obtained through the utilization of the endothermic Boudvard reaction and the exothermic Newman reaction. The Boudvard reaction is as follows:

while the Newman reaction is as follows:

2co+oco +co These reactions are mentioned, identified and explained in Principles of Chemical Engineering by Walker, Lewis, McAdams and Gilliland. With the process of this invention and because of the very rich fuel mixture utilized, these reactions are believed to occur at a very substantial extent. In these reactions oxygen does not directly contact the carbon. Carbon is attacked by carbon dioxide, in the absence of oxygen, absorbing heat. Part of the carbon monoxide formed is oxidized in the Newman reaction giving off heat and carbon dioxide and providing the oxygen free atmosphere required for the Boudvard reaction. Whether or not this theory is correct, large quantities of C0 are generated in the zone 48, and it has been found that is essential that the temperature be maintained at the high value previously mentioned in this zone. If this temperature falls, the CO is not generated but rather large quantities of unreacted carbon egress from the zone 48 and pass out of the furnace and the steam generator. This would be expected with the theory previously stated since the Newman reaction is a very temperature sensitive reaction, and the high temperature previously men tioned must prevail for this reaction to take place in any significant amount.

In any event, applicant has demonstrated this process in a high capacity steam generator having a furnace approximately ft. high and of rectangular cross-section with the dimensions of 25 x 30 ft. This steam generator will produce at maximum load 600,000 pounds of steam per hour with a superheat temperature of 950.

The process of the invention was carried out in this furnace with the improved conditions hereinbefore mentioned being obtained. This was a wet bottom furnace design but with the process of the invention at all loads below three-quarters load, no slag was formed. Complete combustion was obtained with only approximately 10% excess air which is approximately 5% less than that obtained with normal pulverized coal firing of this unit.

In addition to improved flame stability, combustion and slag conditions, control of superheat with varying load may be obtained with the improved process of the invention. This is achieved by adjustably proportioning the air between the zone 50 and the zone 48. Within the zone 48 the air may be adjusted within the 40-80% limits hereinbefore mentioned with regard to the stoichiometric air quantity. The air that is introduced at the zone 50 is only that required to effect complete combustion of the combustible constituents passing through this zone from the zone 48. Thereafter as the air quantity in the zone 48 is reduced, the quantity of air in the zone 50 is increased. This, in turn, causes a greater amount of burning in, the zone 50 and, accordingly, increases the temperature of the combustion gases passing from the furnace and over the heat exchange surface as, for example, the reheater 32. This increase in gas temperature results in an increase in steam temperature egressing from the reheater 32.

In the illustrative embodiment there is provided the temperature responsive device 56 which responds to the temperature of the reheat steam being delivered to turbine 36. This temperature responsive device, in turn, actuates the control mechanism 58 which through motors 60 and 62 control the dampers 54 and 48, respectively.

This automatic 'regulation'is such as to'control, within limits, the reheat temperature to the turbine 36 with the control causing a greater proportion of the air to be delivered to the zone 50 in order to provide for an increase in the temperature of the steam delivered to turbine 36 and vice versa. The regulation is suchas to .maintain the air quantity delivered to the zone 48 within the 40 80% range identified hereinbefore. In this connection it is noted that one of the safety features of this process resides in'the fact that the closing of damper 48 will not cause extinguishment of the flame in the furnace. The reason for this is that the carrier airfor transporting the coal through the transport pipes 40 and delivering it through the burners 38 will necessarily be at least 40% of the stoichiometric air quantity required for the fuel. A less air quantity would fail to transport the fuel.

FIGURE 3 graphically represents the increase in gas temperature that can be obtained through adjustably proportioning the air between zones 48 and 50 in the furnace. This curve indicates that if the air proportioning is changed such that an increased portion is introduced into zone 50, the temperature of the gases leaving the furnace rises.

Accordingly, it will be appreciated that applicant has provided an improved method of operating a steam generator and of burning pulverized fuel in the furnace thereof as well as of controlling the steam temperature developed by the generator.

While I have illustrated and described a preferred embodiment of my invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein Without departing from the spirit and scope of the invention. I therefore do not Wish to be limited to the precise details set forth but desire to avail myself of such changes as fall within the purview of my invention.

What I claim is: I

1. The process comprising introducing pulverized coal and air into a first zone at a ratio such that the quantity of air is above 40% and below 80% of the stoichiometric amount necessary for burning of the fuel, maintaining at said first zone a temperature of at least 1800 F., reacting said air with said coal in said zone to produce CO gas, conveying this gas from said first zone to a second zone, introducing sufiicient air into said second zone to convert said CO to CO reacting said CO with said air to produce said CO conveying said CO from said second zone, imparting heat to a fluid from said first and said second zones and from the gases leaving said second zone.

2. In a steam generator having an elongated furnace with an outlet adjacent one end and with said furnace having disposed on its inner walls heat exchange tubes through which the working fluid of the steam generator is conveyed, a gas passageway extending from said outlet and steam heating tubes in said passageway, the improved method of operation comprising suspension firing said furnace by introducing pulverized coal and air thereinto at a region remote from said outlet, regulating the ratio of fuel to air such that a fuel-rich firing is obtained with the air being between 40 and 80% of the stoichiometric value, maintaining the gas temperature in said region at a value at least 1800" F., introducing air at another region spaced from said first region and intermediate said first region and said outlet, regulating this last-mentioned introduction of air such as to provide generally complete combustion of the combustibles in said other region, and conveying combustion gases from this last-mentioned zone from the furnace through said gas pass thereby imparting heat to the steam in said steam heating surface.

3'. The process of claim 2 including adjustably proportioning the air between the first mentioned and second mentioned regions to adjustably regulate the temperature of the gases traversing said steam heating surface and accordingly the temperature of the steam heated in said surface.

4. The improved processcomprising providing a plurality of confined fluid streams so disposed as to define a combustion zone, introducing pulverized coal and air into said zone and reacting said air with said coal to produce heat and CO gas, regulating the ratio of coal to air thus introduced such that an extremely coal-rich reacting mixture is produced with the air being less than and more than 40% of the stoichiometric value, imparting heat from said zone to said confined streams in such amount and providing suflicient fuel within said zone such that the temperature in the zone is maintained at not less than 1800 F., conveying the gases thus produced by such reaction through a predetermined region in said zone, introducing sufiicient air at said region and mixing the same with said gases so as to produce complete combustion with the gases leaving this region containing little or no burnables, imparting heat from this region to said confined streams, and conveying the gases from this region to a point of disposal.

5. The process of claim 4 including passing the gases leaving said zone in indirect heat exchange relation with a fluid to heat the same, adjustably proportioning the air introduced into said region and into said zone along with said pulverized coal to regulate, within limits, the temperature of the gases imparting heat to this last-mentioned fluid to thereby regulate the heat imparted to this fluid.

6. In a steam generating organization having a furnace into which pulverized coal and air are introduced at a first location and whereby gases are produced, said furnace having a second location downstream with regard to gas flow of said first location with air being introduced into said second location, means conveying the gases thus produced to a point of disposal,- heat exchange means forming part of said steam generator disposed so that it is heated by the heat evolved in said first and said second locations and by the hot gases generated at these locations, said heat exchange means being such that steam is generated therein, steam heating means connected to receive this steam and including means disposed such that the gases after leaving said second zone traverse this last-named means, the improved process characterized by regulating the ratio of pulverized coal and air introduced into said first region such that the air is less than 80 and greater than 40% of the stoichiometric amount, introducing suflicient air into said second region such that complete combustion is obtained, maintaining the temperature in said first region at least at 1800 F., regulating the temperature of said steam by regulating the ratio, within the aforesaid limits, of the air introduced into said first and said second regions.

7. The improved firing process for firing pulverized coal into a high capacity furnace and burning the same in suspension therein and which process reduces the slag formation and accordingly corrosion in the furnace and provides fora more stable burning therewithin including introducing the pulverized coal and air in an extremely coal-rich mixture in a first zone in the furnace with the air being less than 80 and greater than 40% of the stoichiometric value and such that substantial quantities of CO gas are produced in said zone, conveying the gases produced in said first zone through a second zone and introducing into said second zone sufiicient air so as to provide complete combustion without there being large excess quantities of air, maintaining the temperature of said first zone at a value at least 1800 F.

8. In a vapor generator having a furnace with heat exchange tubes on the wall thereof with these tubes forming part of the heat exchange surface of the vapor generator by means of which superheated steam is produced and with said furnace being fired with pulverized coal the improved process whereby slag formation within the furnace is reduced and accordingly corrosion is reduced and which provides for a more stable burning within the furnace including introducing pulverized coal and air in an extremely rich coal mixture in a first zone in the furnace with the air being less than 80 and greater than 40% of the stoichiometric value, reacting this mixture so as to produce substantial quantities of CO gas in said zone while maintaining the temperature in the zone at not less than 1800 F., conveying the gases thus produced in said first zone through a second zone, introducing into said second zone suflicient air so as to provide complete combustion of the burnables within the gases without there being large excess quantities of air, conveying the etfiuent from said second zone over steam heating surface to heat the steam to a desired high value, measuring the temperature of this steam and in response thereto regulatingly proportioning the air between said first and said second zones while maintaining the air introduction into said first zone within the aforementioned limits to thereby maintain said steam at a predetermned value.

References Cited UNITED STATES PATENTS 2,416,053 2/1947 Grossman 122235 2,820,438 1/ 1958 Andrews et al. 122-479 2,907,288 10/ 1959 Blornquist 110-28 3,048,131 8/ 1962 Hardgrove 110-72 FOREIGN PATENTS 947,522 1/ 1964 Great Britain.

CHARLES J. MY'HRE, Primary Examiner. 

1. THE PROCESS COMPRISING INTRODUCING PULVERIZED COAL AND AIR INTO A FIRST ZONE AT A RATIO SUCH THAT THE QUANTITY OF AIR IS ABOVE 40% AND BELOW 80% OF THE STOICHIOMETRIC AMOUNT NECESSARY FOR BURNING OF THE FUEL, MAINTAINING AT SAID FIRST ZONE A TEMPERATURE OF AT LEAST 1800*F., REACTING SAID AIR WITH SAID COAL IN SAID ZONE TO PRODUCE CO GAS, CONVEYING THIS GAS FROM SAID FIRST ZONE TO A SECOND ZONE, INTRODUCING SUFFICIENT AIR INTO SAID SECOND ZONE TO CONVERT SAID CO TO CO2, REACTING SAID CO WITH SAID AIR TO PRODUCE SAID CO2, CONVEYING SAID CO2 FROM SAID SECOND ZONE, IMPARTING HEAT TO A FLUID FROM SAID FIRST AND SAID SECOND ZONES AND FROM THE GASES LEAVING SAID SECOND ZONE. 